1. Field of the Invention
The present invention relates generally to III-V semiconductor devices, and, more particularly, to a unique bandgap profile employed in a graded superlattice.
2. Description of Related Art
It is desired to improve the output characteristics of a double heterojunction bipolar transistor (DHBT), which are attributed to problems related to the conduction band discontinuity in the base-collector (BC) junction. Improvement by conventional means has been done by one of the following approaches:
(1) Lowering the conduction band spike to below that of the conduction band minimum in the base, e.g., GaInAs, by incorporating various relatively heavily doped spacers between the base and the collector. However, this approach has the disadvantages of lower breakdown voltage and higher BC junction capacitance; and
(2) Grading the BC (GalnAs-InP) junction linearly through the quaternary alloy Ga.sub.x In.sub.1-x P.sub.y As.sub.1-y. However, this approach has the disadvantage that while the electrons see a smaller potential barrier in the BC junction, features, such as barriers, in the conduction band cannot be eliminated completely. A linear grade by itself is never optimal and conduction band features remain.
The use of an InP collector in an AllnAs/GalnAs/InP DHBT significantly improves the output characteristics over that of a single heterojunction bipolar transistor (SHBT) with a GaInAs collector, primarily, a higher breakdown voltage between the open-circuited collector and emitter, (BV.sub.ceo). However, the existence of a conduction band discontinuity (conduction band minimum of InP minus conduction band minimum of GaInAs) of .about.0.25 eV has been attributed to causing a higher turn-on voltage (V.sub.ce,sat) and "kinks" in the output (I.sub.c vs. V.sub.ce) characteristics, among other undesirable problems.
Thus, a need remains for providing a DHBT device design which results in a completely featureless conduction band edge, free of the undesirable barriers found in the prior art DHBT devices.